S-Adenosylhomocysteine hydrolase is the only enzyme in eukaryotic cells for the removal of AdoHcy, the end-product of biological transmethylation reactions, and therefore the enzyme is critical for the regulation of AdoMet-dependent methylations. Several approaches are being used to determine structure and function of this enzyme. 1) Structure Determination: We have cloned the cDNA for the enzyme from-both rat liver and D. discoideum cDNA libraries, and the rat liver enzyme has been expressed in E. coli. The amino acid sequence is highly conserved between species suggesting that much of the sequence is required for enzyme function. The putative NAD binding site has been identified by homology to several dehydrogenases and by site-directed mutagenesis of specific amino acids within this region. Other amino acid residues involved in catalytic activity and substrate binding are being examined. 2) Genomic Organization and Expression: AdoHcy hydrolase is expressed at a very high level in liver, and is also present at lower levels in all tissues examined. The expression of the AdoHcy hydrolase MRNA in different tissues and the genomic organization of the AdoHcy hydrolase gene in rat are under investigation. 3) Biological Effects: The inhibition of the AdoHcy hydrolase has a number of important pharmacological effects. A large number of adenosine and adenosyl-homocysteine analogs have been examined for their ability to function as inhibitors and/or substrates of AdoHcy hydrolase. In vivo these adenosine analogs have a wide, range of biological activities, including antiviral activity against several RNA and DNA viruses, inhibition of leukocyte chemotaxis, and modulation of cell differentiation.